1. Field of the Invention
This invention relates to a punch apparatus and method for punching holes in thin sheet material and, more particularly, to a method and gang-punch apparatus containing a plurality of punches in a fixed pattern and which apparatus is programmable to activate certain punches to form holes in a desired hole pattern in the thin sheet material, such as a greensheet.
2. Description of Related Art
In the manufacture of multilayer ceramic (MLC) substrates for integrated circuit semiconductor package structures, a plurality of green ceramic sheets (commonly termed greensheets) are formed by doctor blading a slurry containing a resin binder, a particulate ceramic material, solvents and a plasticizer, drying the doctor bladed sheet and cutting it into appropriate size sheets. Through-holes, commonly termed vias, are then punched in the greensheets to facilitate forming electrical interconnections through the sheet. Electrically conductive paste is deposited in the holes and in appropriate patterns on the surface of the greensheets, the sheets stacked and laminated, and the assembly subsequently fired at a sintering temperature.
The formation of the vias in the ceramic material is accomplished in its green state. Prior to being fired, the greensheets are easily deformed and the precision with which the via holes are formed is greatly dependent upon the use of an apparatus which minimizes distortion or disruption to the structural integrity of the greensheet other than forming the via hole. With the size of electrical components constantly being reduced through technical improvement, the positioning of the vias in ceramic greensheets is becoming ever more restrictive. Further, with the increase in the quantity of the electronic circuits that may be positioned and packaged on a small area of the ceramic substrate, a requirement for a larger number of via holes in the greensheet is evident.
Large numbers of holes must be punched into a greensheet with a high degree of precision in order to properly position the holes. At the same time, this punching operation must occur very rapidly in order to produce the large numbers of greensheets that are necessary in the mass production of electronic components.
A significant portion of the costs associated in manufacturing ceramic packages can be attributed to punching vias in greensheets. A number of factors play a major role in accounting for these costs such as: 1) the high cost for punch die sets customized to a specific product; 2) the long lead-time associated with punch die set fabrication precludes early introduction of a new product; 3) non-customized automated punch die sets have very low through put for a non-grid product; and 4) automated punch/die sets punches step repeat numerous times to punch a single sheet resulting in low throughput.
One method for punching greensheets is using a gang-punching apparatus. A gang-punch is a punch/die set where all the punches for a particular ceramic greensheet layer are rigidly and permanently mounted in a fixed plate. The gang-punch is used to punch the entire sheet (layer) with a single stroke. A gang-punch has very high throughput and is cost effective for very high volume products.
Unfortunately, the use of a gang-punch apparatus has drawbacks since the fabrication of dedicated gang-punches with fixed patterns of via holes are not practical for engineering changes (E.C. 's) or multiple patterns because the same apparatus cannot be easily modified to form different via hole patterns in ceramic greensheets having a different via pattern thereby requiring separate gang-punch apparatus for each E.C. or pattern desired. Additionally, the turn-around time for a new gang-punch/die set is lengthy and high via density is unattainable with single punch heads and usually require multiple punch heads with staggered punches to punch tight grids. The cost is also prohibitive for the punches since for a typical pattern (up) that is specified nine (9) times on each greensheet (9-up) and with each pattern having 2000 (or more) vias, such a 9-up layer would require a gang punch apparatus with a total of 18,000 punches. For a multilayer ceramic substrate composed of ten (10) layers, with each layer requiring a separate gang punch apparatus with 18,000 punches, 180,000 punches would be required to populate these apparatus. At a typical cost of $1 per punch, it would cost $180,000 for the punches alone. Current technology limits for single stroke gang-punching is a minimum punch spacing of about 16 mils.
A number of attempts have been made to provide a gang-punch apparatus which will economically and efficiently provide the means to make via openings in greensheets. One type apparatus utilizes an interposer mask which contains hole openings where holes are desired to be punched. In these apparatus, a plurality of punch elements arranged in a grid are positioned over the greensheet which is covered by the interposer mask. When the punch elements contact the interposer mask as the punch heads are moved downwardly, the holes will be punched where the openings occur since the punch element would pass through the openings in the interposer mask and through the ceramic greensheet. In other areas covered by the interposer mask, i.e., where holes are not desired, the interposer mask will cause the punch element to be retracted into the punch head. Such apparatus are shown in IBM Technical Disclosure Bulletins, Vol. 16, No. 12, May 1974; Vol. 19, No. 4, September 1976 and Vol. 26, No. 10A, March 1984. Other punch apparatus are shown in U.S. Pat. Nos. 4,872,381; 4,425,829; 5,024,127; 5,090,284; and 5,233,895. In U.S. Pat. No. 4,872,381, supra, for example, a programmable magnetic repulsion punching apparatus is shown. A progressive gang-punch is shown in IBM Technical Disclosure Bulletin Vol. 15, No. 2, July 1972 wherein a gang-punch is shown utilizing steel balls on the surface of the punch head at locations of punches to be punched which when a roller traverse the top of the punch head only vias are punched where steel balls are located. The disclosures of the above references and patents are hereby incorporated by reference.
Bearing in mind the problems and deficiencies of the prior art, it is therefore an object of the present invention to provide a programmable gang-punch apparatus for punching selected hole patterns in thin sheet material such as greensheets.
It is an additional object of the invention to provide a programmable gang-punch apparatus which may be fabricated using laser or photoetch techniques.
It is further object of the present invention to provide a method for punching via holes in greensheets using the gang-punch pin apparatus of the invention.
It is another object of the invention to provide greensheets and other substrates punched with through holes using the apparatus and method of the invention.
Other objects and advantages of the present invention will be readily apparent from the following description.